Polyvinyl alcohol (hereinafter, also referred to as PVA), which is excellent in transparency, oil resistance, chemical resistance, and gas (e.g. oxygen) barrier properties, has been widely used as a packaging material.
In recent years, PVA has been often used as a packaging material for foods, drugs, industrial chemicals, agrochemicals, or the like whose characteristics are largely affected by oxidation degradation.
In addition, PVA, which is highly water-soluble, has been used as a provisional protection film or sheet to protect products or members. For example, in cases where PVA is used as a provisional protective film in protection of surfaces during metal processing, protection of rubber members during vulcanization, or protection of surfaces of resin molded products, a separating step or disposal step of such a protective film can be skipped because the film can be easily removed by water, warm water, or hot water.
Patent Literature 1, for example, discloses such a PVA film. The film contains several types of specific surfactants and thus has excellent optical properties (significantly suppressed optical stripe or optical color shading) and can show excellent blocking resistance.
Unfortunately, however, a roll of a PVA film containing a surfactant has recently been found to significantly turn yellow after storage for several months at room temperature. Although yellowing of a PVA film has little effect on the properties such as mechanical strength, extensibility, or haze, contents packaged with a PVA film appear yellowish, or a light beam transmitted through a polarizing film including a PVA film as a material appears yellowish. Such appearances may give a bad impression to consumers or users.
Patent Literature 2 discloses a PVA film which contains polyvinyl alcohol and 0.001 to 1 part by mass of a surfactant based on 100 parts by mass of the polyvinyl alcohol, and is controlled to have a pH in a predetermined range when dissolved in water. Such a PVA film is less likely to turn yellow even after storage for several months at room temperature.